This invention relates to high-temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electric automobiles, storage of energy generated during intervals of off-peak power consumption and various other applications. It is particularly applicable to electrochemical cells that employ metal sulfides as positive electrode active materials and lithium alloys such as lithium-aluminum or lithium-silicon as the negative electrode active material.
A substantial amount of work has been done in development of these types of electrochemical cells and their electrodes. Examples of such high-temperature cells and their various components are disclosed in U.S. Pat. Nos. 3,887,396 to Walsh et al., entitled "Modular Electrochemical Cell," June 3, 1975; 3,907,589 to Bay and Martino, entitled "Cathodes for a Secondary Electrochemical Cell," Sept. 23, 1975; 3,933,520 to Gay and Martino entitled "Method of Preparing Electrodes with Porous Current Collector Structures and Solid Reactants for Secondary Electrochemical Cells," Jan. 20, 1976; and 3,941,612 to Steunenberg et al., entitled "Improved Cathode Composition for Electrochemical Cell," Mar. 2, 1976. The method of the present application is in particular an improvement to that disclosed in allowed U.S. patent application Ser. No. 510,840, now U.S. Pat. No. 3,947,291, to Yao and Walsh, entitled "Electrochemical Cell Assembled in Discharged State," filed Sept. 30, 1974. Each of these patents and patent applications is assigned to the assignee of the present application.
Previous methods for preparing electrochemical cells in uncharged state have been limited in the cell capacity that can be provided in small and light-weight cells. One limiting factor has been the inability to completely discharge lithium-aluminum negative electrodes. A small percentage of the lithium becomes bound within the alloy and is not readily released on discharge of the cell under normal operation. In uncharged cells where the active material is included as the cell reaction product, e.g., lithium sulfide, the positive and negative electrodes will have nearly equal capacities. Additional capacity can be included in the negative electrode by assembling it in a partially charged state. For instance, within a cell in which a 50 atom % lithium and aluminum each is contemplated, the initial lithium electrode composition can include an alloy of about 5 to 10 atom % lithium in, for instance, aluminum. However, in assembling a partially charged cell, certain advantages are lost in that reactive lithium or lithium-aluminum alloy must be handled in an inert gas atmosphere. The completely uncharged cell, on the other hand, can be assembled within a dry air atmosphere.
Therefore, in view of those shortcomings in prior art electrochemical cells, it is an object of the present invention to provide an improved positive electrode composition in the uncharged state.
It is a further object to provide an uncharged positive electrode composition that permits electrochemical charging to form negative electrodes of greater capacity than the positive electrode within the electrochemical cell.
It is also an object to provide an improved uncharged positive electrode composition that can be electrochemically charged to provide an electrochemical cell of increased capacity.